Innovation inside the reactor

How state-of-the-art reactor internals helped debottleneck a Total hydrocracker

After two hydrocracker cycles that were challenged by pressure drop, reliability issues and short cycle lengths, the technologists at the Total refinery in Normandy, France, asked Criterion Catalysts & Technologies (Criterion) and Shell Global Solutions to help resolve these issues.

The unit was not a Shell design but it was imperative that the refinery should address the root cause of its problems, as Total also needed to increase the hydrocracker capacity to produce more valuable middle distillates.

By working together, the three companies have been able to identify the issues that were causing the hydrocracker’s production downtime and improve its operability.

The hydrocracker, which was already the largest in Europe and can now process up to 10,000 tonnes per day, was built in 2007.

A single reactor configuration, it originally featured six catalyst beds.

The first four were to pretreat the feed to remove nitrogen, sulphur and aromatics, and the last two beds hydrocracked the intermediate product into ultra-low-sulphur diesel, kerosene and naphtha.

“A lower-quality vacuum gas oil can affect the performance of the hydrocracking catalyst by accelerating its deactivation rate,” he says.

“We found that one reason why the unit could not achieve its target cycle length was because it was experiencing a high radial temperature spread over the pretreatment catalyst beds.

This is common when conventional distribution trays are used.

The combination of higher radial temperatures and the higher deactivation rate caused by the lower feed quality resulted in an unpredictable and hard to manage cycle length.

“Poor thermal distribution can also lead to hot spots in the cracking beds,” he continues.

“This can cause temperature runaways and change the yield pattern, which means that the unit may make more gas compared with during normal temperature distribution.”

Edwin Maas, Principal Engineer, Shell Global Solutions, adds that there were further challenges.

Total’s original request was for Shell Global Solutions and Criterion to replace the reactor internals on a like-for-like basis.

However, closer inspection revealed several other aspects of the reactor’s design were limiting its performance.

For instance, the catalyst support grids had a very low net free area.

“The catalyst support grids were fouling extremely quickly because they were actually acting as filters,” Maas says.

“Moreover, we identified that beds two and three could be combined into one without affecting reliability.

This meant that because one set of interbed internals could be omitted there was the potential to load much more catalyst into the reactor, which would enhance capacity and cycle length,” he concludes.

By working together, the three companies have been able to identify the issues that were causing the hydrocracker’s production downtime and improve its operability.

After a series of technical discussions during which Shell Global Solutions and Criterion demonstrated why the unit was not achieving the desired performance and the potential value that could be captured, Total gave the go‑ahead to remove everything inside the vessel and completely rebuild it.